Counter-balancing mechanism

ABSTRACT

A counter-balancing mechanism comprising a support ( 1 ) for supporting an article to be tilted about a horizontal axis; a cam ( 4 ) movable with the support, the cam surface of which varies in a direction normal to the plane of rotation of the support; and a cam follower ( 9 ) loaded by a resilient means ( 12 ) whereby, as the support rotates, the cam follower and resilient means causes a return force to be exerted to substantially conteract the torque caused by the rotation of the support. Preferably, a means for varying the return force in accordance with a load on the support is provided, wherein the cam and resilient means is able to rotate about a center offset from the central axis about which the support rotates and including means for setting a variable reaction point, against which the cam follower force reacts, at or offset from said central axis, thereby to cause the return force exerted due to rotation of the support about a particular angle to be varied.

[0001] This invention relates to a counter-balancing mechanism. Inparticular, but not exclusively, it relates to a counter-balancingmechanism forming part of a pan/tilt head, such as a fluid head, formounting on a tripod and supporting a camera or other equipment whichcan be tilted about a horizontal axis.

[0002] There is often a requirement for camera equipment to be tiltedabout a horizontal axis using a head which can hold the equipmentbalanced at any desired angle about the horizontal axis. Whilst a manualmechanism can be used, in which the camera is set at the desired angleand then the head is clamped at that angle, this is unsatisfactory andcumbersome and is not very useful for professional purposes.

[0003] Many counter-balancing mechanisms have been proposed in the pastbut up to now it has been found very difficult to obtain acounter-balancing mechanism which effectively counter-balances a load(which can be fairly substantial, up to around 50 kilograms for a camerafor example) at virtually any desire angle. Referring to FIG. 1, if acamera of load L is rotated about a horizontal axis A through a desiredangle then the torque at any angle is equal to the load L times thehorizontal distance L of the load from the axis and this increasesgenerally sinusoidally as the angle from the vertical increases. Thecounter-balancing mechanism is required to counteract this and, since itis not a linear function, this is difficult to achieve within a packageof acceptable size/mass.

[0004] Furthermore, allowance must also be made first for variations inthe weight of an item to be placed upon the support and tilted, andsecondly for variations in the position of the centre of gravity of anitem (for example, a camera mounted on a tripod may have its centre ofgravity which is not coincident with the point which will be mounteddirectly above the centre of the tripod).

[0005] The present invention arose in an attempt to provide an improvedcounter-balancing mechanism.

[0006] According to the present invention in a first aspect there isprovided a counter-balancing mechanism comprising a support forsupporting an article to be tilted about a horizontal axis; a cammovable with the support, the cam surface of which varies in a directionnormal to the plane of rotation of the support; and a cam followerloaded by a resilient means whereby, as the support rotates, the camfollower and resilient means causes a return force to be exerted tosubstantially counteract the torque caused by the rotation of thesupport.

[0007] The resilient means is preferably on or more springs.

[0008] Preferably, a means for varying the return force in accordancewith a load on the support is provided, wherein the cam and resilientmeans is able to rotate about a centre offset from the central axisabout which the support rotates and including means for setting avariable reaction point, against which the cam follower force reacts, ator offset from said central axis, thereby to cause the return forceexerted due to rotation of the support about a particular angle to bevaried.

[0009] According to the present invention there is further provided acounterbalancing mechanism or a tilt head comprising any one or more ofthe novel features herein described.

[0010] Embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

[0011]FIG. 1a and FIG 1 b show a load pivoting about a pivot point;

[0012]FIG. 2 shows schematically a side view of part of a supportapparatus;

[0013]FIG. 3 shows a partial perspective view of the apparatus;

[0014]FIG. 4 shows a side view of part of the apparatus;

[0015]FIG. 5 shows a cam profile and follower;

[0016]FIG. 6(a) and (b) shows the affect of rotating the cam followerabout an axis which is off-set from the axis of rotation of theplatform; and

[0017]FIG. 7 shows an exploded view of part of an embodiment of theinvention.

[0018] As described above, with reference to FIGS. 1a and 1 b, as a loadon a platform is tilted about an axis, the load introduces a varyingtorque. The invention arose in an attempt to provide a mechanism for atiltable platform, such as tilt/pan head for camera or otheraudio-visual equipment, which can counter-balance this effect at anydesired tilt angle.

[0019] Referring to FIG. 2, a tilt/pan head comprises a platform 1 uponwhich apparatus to be supported, such as a camera, is mountable. Theplatform 1 is mounted upon two spaced rectangular plates 2, 3. Plate 3is shown in more detail in FIG. 7, plate 2 is similar. Plate 2 isprovided with a cam 4 which extends from one face towards the otherplate forming a generally annular axial projection as shown in FIGS 2and 3. The cam may be integral with the plate or affixed to it. Theheight of the cam varies radially around the cam. Plate 2 is mounted sothat it can rotate about a horizontal axis relative to a fluid filedgenerally cylindrical body 5. Relative rotation occurs about bearingsbetween these, which bearings are not shown for clarity. Similarly,plate 3 rotates relative to a cylindrical body 6 and a hub 22 and againbearings are provided. Bodies 5 and 6 are connected to a commonstructure 7 which is mounted to, or indeed could form part of, a tripodor other stable structure upon which the head can be mounted. Where thehead is a tilt and pan head, the structure 7 may be arranged to rotateabout a vertical axis so that rotation in both the horizontal andvertical plane is achieved. This range or rotation is usually desirable.

[0020] A further cylindrical body (or drum) 8 is sized so that it cansit within the annular region defined by the cam 4 and has, on itlateral surface, a cam follower 9. This generally comprises a stud, pinor projection 10 extending from the outer surface of body 8 and anannular ring coaxial with the pin and adapted to freely rotate relativethereto 11, the ring being able to slide relative to the top surface ofthe cam 4.

[0021] Body 8 is generally solid apart from being provided with aplurality of bore holes 14 which open at the surface of the body nearestto hub 22. A plurality of springs 12 are mounted, one in each of thebore holes and these are mounted, or simply supported, at their otherend by the hub 22. The plate 3 rotates relative to this. Other types ofresilient means may be provided but preferably these are springs whichare preferably pre-stressed. In one non-limiting embodiment, twentysprings are provided, the rating of each spring being about 9.72 N/mm.

[0022] The bores in member 8 are simply to provide locations for thesprings and the springs may instead simply be mounted against one end ofthe cylindrical body 8.

[0023] In use, the head assembly 13 is based upon a tripod or otherstable structure and a camera or other apparatus to be tilted is affixedto platform 1 by a suitable means. When the apparatus is tilted, the cam4 also rotates. The cam follower is pushed by the springs 12 against thecam and thus the cam follower is moved with a longitudinal component ofmotion by the action of the cam. This causes the springs 12 to becompressed to a further or lesser extent and the hub 22 produces anequal and opposite return force on body 8 and therefore causes the camfollower 9 to exert a greater or lesser returning force against the cam4. As is shown schematically in FIG. 5, a return force F in a directionwhich is not normal to a cam surface also causes a force having acomponent at 90° to the force F and which serves to resist movement ofthe cam. It is this resisting force, which varies as the return force onthe spring varies as the cam is moved longitudinally backwards andforwards, which serves to counter-balance the varying torque of the loadtilting.

[0024] Since the change in torque as the platform rotates is not linear,the shape of the cam has to take account of this non-linearity. The camprofile, in terms of height and degree of rotation has to take thefollowing into account.

[0025] (a) The moment given by the load coming off-centre;

[0026] (b) Change in effective length of lever due to the load pivotingabout the axis;

[0027] (c) Spring energy due to compression given by cam height; and

[0028] (d) Gradient of the cam.

[0029] The spring energy depends upon the preload and additionalcompression given by the cam height. It may be noted that cam height andgradient are interactive. Preferably, the profile is calculated for eachpoint on the cam to within, say, one tenth of a degree but may beprovided in other (eg larger) increments or by formula if produced oncomputer aided machinery and this can be done by those skilled in theart. In one example, for a particular load and assuming that the axis ofthe centre of gravity of the load coincides with the central axis of thetilting apparatus, one set of cam and cam rise angles varies with camangle as follows: TABLE 1 cam rotation angle cam height cam gradient 0 00 5 0.302058 5.66776 10 1.151864 10.448357 15 2.441784 13.963848 204.047112 16.297206 25 5.857201 17.704133 30 7.785164 18.431862 359.766036 18.667698 40 11.75182 18.543012 45 13.70694 18.149181 5015.6049 17.551556 55 17.42597 16.799146 60 19.15571 15.930768 65 20.783914.978767 67.9 21.67896 14.39946 70 21.70921 14.39946

[0030] This assumes a payload of 40 Kg, load radius of 250 mm, use of 20springs each rated as 9.72 N/mm having free length of 89.9 mm andminimum length of 44.27 mm.

[0031] The cam followers are, in one embodiment of the invention,constrained so that the cam rotates relative to them, in order for thefollowers to move around the cam.

[0032] Although one follower is shown in FIG. 2, a plurality ofequi-spaced followers is highly desirable and will generally be used, inorder to balance forces about the axis, in keeping with good practice.

[0033] Bodies 8 and 22 are required to be torsionally rigid with regardto each other, whilst allowing for relative axial movement. This can beachieved by many means such as keys, splines or similar. However, inorder to avoid detrimental backlash or axial friction either chordallinks or the method as follows may be used. In the present preferredembodiment, a further pair of projections 20, 21 (FIG. 4) are providedon cam follower body 8 and, in use, these impinge upon a peripheralaxial projection 22 b from hub 22 which may require a cut-out 22 a toclear axial movement of cam follower 10. The elements 20 and 21 may beof similar construction to the cam follower, having a projection frombody 8 and a rotatable disk so that they, and therefore body 8, canslide longitudinally relative to hub 22. If, as may be preferred for abalance of forces, more than one projection is provided, then anequivalent number of associated pairs of elements 20 and 21 (camfollowers) may be provided, being equispaced.

[0034] The cam profile, and/or spring rate is chosen in the abovedescribed embodiment for a load of a particular weight and which ismounted with its centre of gravity in a particular position relative tothe support mechanism so as to provide equal and opposite return forcesto counterbalance tilt of the load. In practice, it is desirable to beable to accommodate loads of different weights or of variable centres ofgravity, which may well not overlie the centre of the apparatus.Accordingly, in preferred embodiments of the invention, a mechanism isincluded to provide for this adjustability.

[0035]FIG. 4 shows parts of the apparatus which may be termed an ‘energypack’ 40. This comprised drum 8, with followers 10, 20, 21, hub 22 and alever 27 with shaft 23 that extends through plates 22, 26 and into plate2. Thus the ‘energy pack’ can rotate about an axis which is offset fromthe axis of rotation O of the platform. Hub 22 (and therefore the camfollowers and springs) rotates with shaft 23, as shaft 23 has a key 20(FIG. 7), which fits into a keyway 25 in the central hole in hub 22. Thecam is also mounted offset from the centre of plate 2 and so the axis ofrotation of the cam and resilient means is offset from the axis ofrotation of the support (being coaxial with bearing 26).

[0036] FIGS. 6(a) and 6(b) illustrate the effect of this. In FIG. 6(a)point O is the centre of tilt rotation, D is the centre of the energypack and E is the reaction point for the energy pack. A mechanism forachieving this is described further below. The load of mass K and ispositioned such that its centre of gravity acts at point G which isspaced a distance M, given by and commensurate with α, from the centreof tilt rotation O.

[0037] As the load is tilted by an angle α then the cam followers arerotated around the cam by angle β°, due to being constrained by reactionpoint E. The cam/spring configuration is designed to provide a reactionequal and opposite to the load torque KM for any angle of α between plusor minus 90°. As described, this can be precisely calculated if the loadand the radius OG is constant. To accommodate changes in the load K orthe radius OG (and therefore M) the distance A may be adjusted. That is,the position of the reaction point may be adjusted. For a reduced momentKM, A needs to be reduced and this reduces the FIG. 6(b) where point Ehas been moved such that it is a distance of A′ from the centre of tiltrotation O.

[0038] As is seen from a comparison of FIG. 6(a) and FIG. 6(b) ifdistance A is reduced from a to A′ then angle β also changes from β toβ′ for the same angle α. The practical effect of this is the camfollowers are acted upon by a different part of the cam and thereforeexert a different return force and therefore counterbalancing force forthe same angle of tilt. Thus, a light load at a tilt of α° may use thatpart of the cam which will be used by a heavier load at a differentangle α plus or minus Δα. For example, a light load at, say, 60° tiltmay use that part of the cam which would be used by a heavier load at,say, 30°.

[0039] The cam calculation is required to account for variation inlength ED as α changes, for a fixed distance A. As shown in FIG. 7, anadjustable reaction point E is achieved by providing a lever 23 whichpasses through a bearing 24 which is generally central within plate 3(FIG. 2) and drives hub 22 via key 40 and keyway 25. Lever 23 extendsthrough a plate 6 (FIG. 2) which is mounted with its centre off axisfrom the centre of plates 3 and 2 and able to rotate about bearing 26.The lever has a generally U-shaped portion 27 having two arms 28 and 29which hold between them a moveable key 30 which is slidable within thegap between the arms. A rear member 31 fixed within plate 6 is providedwith a slot or cutaway 32 in which a pin is mounted upon a threaded bolt34 or similar to move relative to the bolt away from and towards thecentre of body 31. This is preferably adjusted from the other side ofthe plate by a simple rotating know for example which can move, by awell-known mechanism, the pin relative to the bolt. In order to adjustthe point against which the energy pack reacts, the pin 33 is moved to asuitable radial position and this moves the key 30 relative to the lever23. This adjustment can be done on a trial and error basis with eachdifferent load or, more preferably, the device can be calibrated so thatonce the load is known, and/or the offset of the mass of the load fromthe centre is known, the position of E can be established.

1. A counter-balancing mechanism comprising a support for supporting anarticle to be tilted about a horizontal axis; an annular cam movablewith the support, the cam surface of which varies in a direction normalto the plane of rotation of the support; and a cam follower loaded by aresilient means whereby, as the support rotates, the cam follower andresilient means causes a return force to be exerted to substantiallycounteract the torque caused by the rotation of the support, wherein ameans for varying the return force in accordance with a load on thesupport is provided, wherein the cam is able to rotate about its ownaxis which is offset from the axis of rotation of the support andincluding means for setting a variable reaction point, against which thecam follower force reacts, at or offset from the axis of rotation of thesupport, thereby to cause the return force exerted due to rotation ofthe support about a particular angel to be varied.
 2. Acounter-balancing mechanism as claimed in claim 1, comprising an ‘energypack’ adapted to rotate about an axis which is offset from the axis ofrotation of the support, the energy pack comprising a drum having atleast one cam follower, a hub and a lever arranged to enable the energypack to rotate about an axis offset from the axis of rotation of thesupport.
 3. A counter-balancing mechanism as claimed in claims 1 or 2,wherein the cam is mounted offset so that the axis of rotation of thecam and resilient means is offset from the axis of rotation of thesupport.
 4. A counter-balancing mechanism comprising a support forsupporting an article to be tilted about a horizontal axis; a cammovable with the support, the cam surface of which varies in a directionnormal to the plane of rotation of the support; and a cam followerloaded by a resilient means whereby, as the support rotates, the camfollower and resilient means causes a return force to be exerted tosubstantially counteract the torque caused by the rotation of thesupport, wherein the cam is annular and the or each cam follower isprovided on a body which locates within the annular cam and protrudesoutwardly from the body.
 5. A counter-balancing mechanism as claimed inany preceding claim, forming part of a pan/tilt head.
 6. Acounter-balancing mechanism as claimed in any preceding claim, whereinthe resilient means is one or more springs.
 7. A counter-balancingmechanism as claimed in any preceding claim, wherein the cam forms agenerally annular projection from a body, the projection having variableheight to provide the cam surface.
 8. A counter-balancing mechanism asclaimed in claim 7, wherein the support is mounted upon two spacedplates, one of which bears the cam.
 9. A counter-balancing mechanism asclaimed in claim 8, wherein at least one cam follower is provided on abody which is resiliently mounted against a hub adapted for relativerotational movement with the other of said plates.
 10. Acounter-balancing mechanism as claimed in claim 9, wherein the body fitswithin the region defined by the annular cam and the or each followerprotrudes outwardly from the body.
 11. A counter-balancing mechanism asclaimed in any preceding claim, wherein the cam profile is chosen totake the following into account: (a) the moment given by a loadcoming-off centre; (b) change in effective length of lever due to a loadpivoting about the axis; (c) spring energy due to compression derivedfrom cam height; and (d) gradient of cam.
 12. A counter-balancingmechanism as claimed in any preceding claim, wherein the or each camfollower is contoured so that the cam moves relative to it.
 13. Acounter-balancing mechanism as claimed in any preceding claim,comprising a plurality of equi-spaced followers.
 14. A counter-balancingmechanism as claimed in any preceding claim, including means forcompensation for variation in load or distance of the centre of gravityof the load from a pivot.
 15. A counter-balancing mechanism as claimedin claim 14, comprising a means for selecting a reaction point.
 16. Acounter-balancing mechanism as claimed in claim 15, wherein a leverforming part of an energy pack forms a keyway and a key is selectablylocated at a variable position within the keyway, to form a variablepivot reaction point.
 17. A counter-balancing mechanism as claimed inany of claims 1 to 2, wherein the cam is annular and the or each camfollower is provided on a body which locates within the annular cam andprotrudes outwardly from the body.
 18. A pan/tilt head, comprising acounter-balance mechanism as claimed in any preceding claim.